Hong Kong booksellers and publishers have started removing politically sensitive books from their shops, according to several media reports. The move was triggered by the latest disappearance of a bookstore employee — one working for a firm producing books critical of Chinese leaders.

Lee Bo, a British national and chief editor of Causeway Bay Books, which produced books on Chinese politics, was last seen on Dec. 30 at the publisher’s warehouse. His wife reportedly withdrew a missing persons report soon after filing it, claiming Lee had traveled to China to voluntarily assist in an unspecified investigation.

Owners of three stores selling books on political topics refused to be publicly interviewed fearing backlash from the mainland, Reuters reported. On Thursday, the Guardian reported that Page One, a major Singapore book retailer with shops in both Hong Kong and mainland China, decided to stop selling politically sensitive books written in Chinese.

Four other employees of Causeway Bay Books and Mighty Current, another publishing house, have disappeared since late last year. Hong Kong Police have said that they continue to investigate while China has yet to clarify on the fate of the men.

The disappearances have fuelled speculation that enforcement officials from the mainland have abducted the booksellers. Hong Kong, which is a special administrative region of China, operates under the "one country, two systems" policy that grant it autonomy and freedom from certain laws that govern the mainland. However, activists have complained of Beijing's growing interference and influence in Hong Kong in recent years.

"Nobody is safe in Hong Kong now," Bao Pu, a prominent publisher in Hong Kong, told Reuters. Bao, who published the secret memoirs of Zhao Ziyang, a former Communist Party general secretary, was openly critical of the events, and told Reuters that he suspected mainland officials.

Albert Ho, a pro-democracy lawmaker, told CNN: "It's a forced disappearance. All those who have disappeared are related to the Causeway Bay bookshop and this bookshop was famous, not only for the sale, but also for the publication and circulation of a series of sensitive books."

U.K. Foreign Secretary Philip Hammond reportedly said he was unable determine the booksellers' whereabouts after raising the case with Chinese and Hong Kong officials during a two-day trip to Beijing.